High-frequency transmission line and electronic device

ABSTRACT

A transmission line portion of a flat cable includes first regions and second regions connected alternately. In the first region, the transmission line portion is a flexible tri-plate transmission line including a dielectric element including a signal conductor, a first ground conductor including opening portions, and a second ground conductor which is a solidly filled conductor. In the second region, the transmission line portion is a hard tri-plate transmission line including a wide dielectric element including a meandering conductor, and a first ground conductor and a second ground conductor which are solidly filled conductors. A variation width of the characteristic impedance in the second region is larger than a variation width of the characteristic impedance in the first region.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a thin high-frequency transmission linefor transmission of a high-frequency signal and an electronic deviceincluding the high-frequency transmission line.

2. Description of the Related Art

Typical examples of a high-frequency line for transmission of ahigh-frequency signal according to the related art include a coaxialcable. The coaxial cable includes a center conductor (signal conductor)configured to extend in one direction (configured to extend in thesignal transmission direction), and a shield conductor providedconcentrically along the outer peripheral surface of the centerconductor.

In recent years, high-frequency devices including mobile communicationterminals have been reduced in size and thickness, and a space forarrangement of the coaxial cable may not be secured in a terminalhousing. In addition, the coaxial cable is hard, and thus not easilycurved or bent to be routed.

Use of high-frequency transmission lines described in InternationalPublication No. 2011/007660 and Japanese Registered Utility Model No.3173143 is drawing attention to solve a problem that occurs when such acoaxial cable is utilized. The high-frequency transmission line islarger in width than the coaxial cable but can be thinned, and thereforeis particularly useful in the case where there is only a small clearancein the terminal housing. In addition, the high-frequency transmissionline has a flexible dielectric element as the base material, and thus isflexible and can be easily curved and bent to be routed.

The high-frequency transmission lines described in InternationalPublication No. 2011/007660 and Japanese Registered Utility Model No.3173143 have a tri-plate strip line structure as the basic structure.

The high-frequency transmission lines described in InternationalPublication No. 2011/007660 and Japanese Registered Utility Model No.3173143 include a flat-plate dielectric element that is flexible andinsulating. The dielectric element has an elongated shape that extendsstraight. A second ground conductor is disposed on a second surfaceorthogonal to the thickness direction of the dielectric element. Thesecond ground conductor has a so-called solidly filled conductor patternin which the second surface of the base material sheet is coveredgenerally entirely. A first ground conductor is disposed on a firstsurface of the base material sheet opposite to the second surface. Thefirst ground conductor includes elongated conductors configured toextend along the longitudinal direction at both ends in the widthdirection orthogonal to the longitudinal direction and the thicknessdirection. The two elongated conductors are connected to each other bybridge conductors disposed at predetermined intervals along thelongitudinal direction and configured to extend in the width direction.Thus, the first ground conductor is shaped such that opening portionswith a predetermined opening length are formed and arranged along thelongitudinal direction.

A signal conductor with a predetermined width and a predeterminedthickness is formed in the middle of the dielectric element in thethickness direction. The signal conductor is elongated to extend in thedirection parallel with the elongated conductor portions of the firstground conductor and the second ground conductor. The signal conductoris formed generally in the center of the dielectric element in the widthdirection.

When the thus configured high-frequency transmission line is viewed inplan (seen from a direction orthogonal to the first surface and thesecond surface), the signal conductor is disposed so as to overlap thefirst ground conductor only at the bridge conductors and be provided inthe opening portions in other regions.

With such a shape, the high-frequency transmission lines havingpredetermined transmission characteristics can be thinned, and can beeasily curved and bent to be routed. The terms “curve” and “bend” asused herein refer to three-dimensional deformation that causes theentire flat-plate surface of the high-frequency transmission lines notto be present on an identical plane. In other words, the terms relate tocausing a bend by a predetermined angle with respect to the flat-platesurface of the high-frequency transmission lines.

However, the high-frequency transmission lines structured as discussedabove have the following problem. FIGS. 14A and 14B are each a graphillustrating the distribution characteristics of characteristicimpedance for explaining the problem with the high-frequencytransmission line structured in accordance with the related art.

In the high-frequency transmission line according to the related art,the first ground conductor includes a plurality of opening portionsprovided along the longitudinal direction. Thus, the first groundconductor and the signal conductor face each other along the thicknessdirection only at the locations of installation of the bridgeconductors. Therefore, a C property (capacitive property) becomeshighest at the positions of the bridge conductors along the longitudinaldirection, and an L property (inductive property) becomes highest in themiddle of the opening portions.

Since the bridge conductors are disposed at predetermined intervals asdiscussed above, the characteristic impedance of the high-frequencytransmission line according to the related art is cyclically varied inaccordance with the interval of installation of the bridge conductors.Setting is made such that a desired characteristic impedance is obtainedfor the overall length of the high-frequency transmission line.

The characteristic impedance is set with the high-frequency transmissionline not curved. Thus, with the high-frequency transmission line notcurved, as illustrated in FIG. 14A, the characteristic impedance isvaried in accordance with the interval of installation of the bridgeconductors, and a desired characteristic impedance Zo is obtained forthe overall length. In this event, the amplitude ΔR0 of the real numberof the characteristic impedance has a constant value.

In the case where the high-frequency transmission line is curved,however, the positional relationship between the signal conductor andthe first ground conductor and the second ground conductor is varied atcurved portions. In this case, the characteristic impedance at thecurved portions is varied. Depending on the curved state, as illustratedin FIG. 14B, for example, the L property in an opening portion may beincreased such that the amplitude ΔR0′ of the real number of thecharacteristic impedance at the curved portions is larger than theamplitude ΔR0 of the real number of the characteristic impedance atnon-curved portions.

The characteristic impedance for the entire high-frequency transmissionline greatly depends on the maximum value of the amplitude of the realnumber of the characteristic impedance. Thus, if the high-frequencytransmission line is curved and the maximum value of the amplitude ofthe real number of the characteristic impedance is varied, thecharacteristic impedance for the entire high-frequency transmission lineis also varied. For example, as illustrated in FIG. 14B, thecharacteristic impedance Z0′ of the entire high-frequency transmissionline with curved portions is different from the characteristic impedanceZ0 of the entire high-frequency transmission line which is not curved.Therefore, the transmission loss of an RF signal may be increased todegrade the transmission characteristics.

SUMMARY OF THE INVENTION

Preferred embodiments of the present invention provide a high-frequencytransmission line with an overall characteristic impedance that ishardly varied even if the high-frequency transmission line is partlycurved in the thickness direction, and an electronic device.

A high-frequency transmission line according to a preferred embodimentof the present invention includes a dielectric element, a signalconductor, and a first ground conductor. The dielectric element is aflexible flat plate elongated in one direction, the flat plate having apredetermined width in a direction perpendicular or substantiallyperpendicular to a longitudinal direction and having a predeterminedthickness. The signal conductor is disposed in the dielectric element,and configured to extend along the longitudinal direction. The firstground conductor opposes the signal line conductor and extends in thelongitudinal direction.

At least one of the signal conductor and the first ground conductor hasa characteristic impedance cyclically varied along the longitudinaldirection, and includes a first region and a second region withdifferent variation widths of the characteristic impedance. Thevariation width of the characteristic impedance in the second region islarger than the variation width of the characteristic impedance in thefirst region.

According to the configuration, the characteristic impedance in thesecond region is dominant in the characteristic impedance of the entirehigh-frequency transmission line. Thus, the characteristic impedance forthe entire high-frequency transmission line is hardly varied if thefirst region is bent, because the characteristic impedance in the secondregion is not varied although the characteristic impedance in the firstregion is varied.

A high-frequency transmission line according to a preferred embodimentof the present invention is preferably configured as follows. In thefirst region of the high-frequency transmission line, the first groundconductor is configured to extend along the longitudinal direction, andconfigured to include two elongated conductors disposed at both ends inthe width direction with a space between each other and configured toextend in the longitudinal direction, and a plurality of bridgeconductors that connect between the two elongated conductors atintervals along the longitudinal direction. The second ground conductoris a conductor arranged to extend generally over the entire other endsurface.

The configuration indicates a specific example of the configuration ofthe first region of the high-frequency transmission line. With such aconfiguration, the first ground conductor is structured such that aplurality of opening portions in which a conductor is not provided arearranged along the longitudinal direction. Thus, the proportion of theconductor occupying the first region is reduced compared to thedielectric element, which enables the first region to be easily curvedor bent.

The second region of the high-frequency transmission line according to apreferred embodiment of the present invention is preferably configuredso as to rotate a phase of a high-frequency signal by a predeterminedamount through transmission between an input end and an output end.

According to the configuration, the second region further functions as aphase adjustment circuit. Thus, the input high-frequency signal iscapable of being transmitted by the high-frequency transmission line tobe output with a predetermined phase.

The high-frequency transmission line according to a preferred embodimentof the present invention is preferably configured as follows. In thesecond region of the high-frequency transmission line, the signalconductor is a conductor line with such a length that rotates the phaseof the high-frequency signal by the predetermined amount. In the secondregion of the high-frequency transmission line, the first groundconductor is a conductor arranged to extend generally over the entireone end surface of the dielectric element.

According to the configuration, the second region of the high-frequencytransmission line is implemented by a distributed constant line. Thus,the second region is configured to function as a phase adjustmentcircuit (delay line) with good frequency characteristics. In addition,the first ground conductor and the second ground conductor are so-calledsolidly filled conductors, which can make the second region harder anddifficult to be curved or bent.

A high-frequency transmission line according to a preferred embodimentof the present invention is preferably configured as follows. Thehigh-frequency transmission line further includes a third groundconductor provided in the second region of the high-frequencytransmission line and arranged generally at the same position as thesignal conductor in the thickness direction, the third ground conductorbeing configured to extend in parallel or substantially in parallel withthe signal conductor with a generally constant gap therebetween, and thethird ground conductor being connected to the first ground conductor athigh frequencies.

According to the configuration, the conductor line constituting thephase adjustment circuit is surrounded by a ground. Thus, thecharacteristics of the phase adjustment circuit are capable of being sethighly precisely, and a desired amount of phase rotation is achievedhighly precisely.

In the high-frequency transmission line according to a preferredembodiment of the present invention, a width of the second region ispreferably larger than a width of the first region.

According to the configuration, the second region has a large area,which makes the second region hard and difficult to be curved or bent.

The high-frequency transmission line according to a preferred embodimentof the present invention is preferably configured as follows. The secondregion of the high-frequency transmission line is provided at aplurality of locations along the longitudinal direction of thehigh-frequency transmission line, and a plurality of such second regionshave generally the same variation width of the characteristic impedance.

According to the configuration, the plurality of second regions whichare not curved or bent have generally the same characteristic impedance,which further stabilizes the characteristic impedance for the entirehigh-frequency transmission line.

A high-frequency transmission line according to a preferred embodimentof the present invention preferably further includes a second groundconductor disposed opposite to the first ground conductor with thesignal conductor interposed therebetween.

Thus, a tri-plate transmission line is provided.

A high-frequency transmission line according to a preferred embodimentof the present invention preferably further include a connector memberfor connection to the signal conductor provided at at least one end inthe longitudinal direction.

According to the configuration, the connector member is configured sothat the high-frequency transmission line is easily connected to anexternal circuit board or the like.

Another preferred embodiment of the present invention provides anelectronic device including any high-frequency transmission lineaccording to the various preferred embodiments of the present inventiondiscussed above, a plurality of circuit boards connected to each otherby the high-frequency transmission line, and a housing that houses thecircuit boards.

According to the configuration, the electronic device includes one ofthe high-frequency transmission lines described above. Use of thehigh-frequency transmission line discussed above allows transmission ofa high-frequency signal between the circuit boards without increasing atransmission loss even if the high-frequency transmission line is usedas curved or bent.

In an electronic device according to a preferred embodiment of thepresent invention, the high-frequency transmission line is preferablybent at least one location in the first region.

According to the configuration, arrangement modes of the high-frequencytransmission line are increased. Further, the characteristic impedanceof the high-frequency transmission line is constant in whicheverarrangement mode is adopted to bend the high-frequency transmissionline. Thus, the transmission characteristics for a high-frequency signalwithin the electronic device are improved.

According to various preferred embodiments of the present invention, thecharacteristic impedance for the entire flat cable is hardly varied evenif the flat cable is partly curved. Thus, a flat cable with stabletransmission characteristics that are not affected by the mode of use isprovided.

The above and other elements, features, steps, characteristics andadvantages of the present invention will become more apparent from thefollowing detailed description of the preferred embodiments withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are each a perspective view illustrating the appearanceof a flat cable according to a first preferred embodiment of the presentinvention.

FIG. 2 is an exploded perspective view illustrating the structure of afirst region of a transmission line portion of the flat cable accordingto the first preferred embodiment of the present invention.

FIG. 3 is an exploded perspective view illustrating the structure of asecond region of the transmission line portion of the flat cableaccording to the first preferred embodiment of the present invention.

FIGS. 4A and 4B are each a graph illustrating the distributioncharacteristics of characteristic impedance along the longitudinaldirection of the transmission line portion of the flat cable accordingto a preferred embodiment of the present invention.

FIGS. 5A and 5B are a side sectional view and a plan sectional view,respectively, illustrating the partial configuration of a portableelectronic device according to the first preferred embodiment of thepresent invention.

FIG. 6 is an exploded perspective view illustrating the structure of asecond region of a transmission line portion of a flat cable accordingto a second preferred embodiment of the present invention.

FIG. 7 is a graph illustrating the distribution characteristics ofcharacteristic impedance along the longitudinal direction of thetransmission line portion of the flat cable according to the secondpreferred embodiment of the present invention.

FIG. 8 is an exploded perspective view illustrating the structure of asecond region of a transmission line portion of a flat cable accordingto a third preferred embodiment of the present invention.

FIG. 9 is an exploded perspective view illustrating the structure of asecond region of a transmission line portion of a flat cable accordingto a fourth preferred embodiment of the present invention.

FIG. 10 is an exploded perspective view illustrating the structure of asecond region of a transmission line portion of a flat cable accordingto a fifth preferred embodiment of the present invention.

FIG. 11 is an exploded perspective view illustrating the structure of asecond region of a transmission line portion of a flat cable accordingto a sixth preferred embodiment of the present invention.

FIG. 12 is an exploded perspective view illustrating the structure of asecond region of a transmission line portion of a flat cable accordingto a seventh preferred embodiment of the present invention.

FIG. 13 is a perspective view illustrating the appearance of a flatcable with a constant width.

FIGS. 14A and 14B are each a graph illustrating the distributioncharacteristics of characteristic impedance for explaining a problemcaused with a flat cable structured in accordance with the related art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A flat cable according to a first preferred embodiment of the presentinvention will be described with reference to the drawings. FIGS. 1A to1C are each a perspective view illustrating the appearance of a flatcable which is a high-frequency transmission line according to the firstpreferred embodiment of the present invention. FIG. 1A illustrates astate in which the flat cable is not curved at all over the overalllength. FIGS. 1B and 1C each illustrate a state in which the flat cableis partly curved, the flat cable being curved more greatly in FIG. 1Cthan in FIG. 1B. In FIG. 1A, the side on which coaxial connectors aredisposed is defined as the upper surface. In FIGS. 1B and 1C, the sideon which the coaxial connectors are disposed is defined as the lowersurface.

FIG. 2 is an exploded perspective view illustrating the structure of afirst region of a transmission line portion of the flat cable accordingto the first preferred embodiment of the present invention. FIG. 3 is anexploded perspective view illustrating the structure of a second regionof the transmission line portion of the flat cable according to thefirst preferred embodiment of the present invention.

A flat cable 60 includes a transmission line portion 10 and coaxialconnectors 61. The transmission line portion 10 has a flat and elongatedshape. The transmission line portion 10 is configured such that a firstregion 100S, a second region 100H, a first region 100S, a second region100H, and a first region 100S are continuously provided along thelongitudinal direction.

Two coaxial connectors 61 are provided, and installed at both ends ofthe transmission line portion 10 in the longitudinal direction. Thecoaxial connectors 61 are installed on the side of a second principalsurface (corresponding to the other surface according to the presentinvention) of the transmission line portion 10 via conversion pedestals62. A center conductor (not illustrated) of the coaxial connector 61 isconnected to an end portion of a signal conductor 40 (see FIGS. 2 and 3)of the transmission line portion 10. Meanwhile, an outside conductor(not illustrated) of the coaxial connector 61 is connected to a secondground conductor 30 of the transmission line portion 10. The coaxialconnectors 61 and the conversion pedestals 62 may be omitted, andarrangements other than the coaxial arrangement may be adopted. In thecase where the coaxial connectors 61 and the conversion pedestals 62 areomitted, portions of the signal conductor 40 around both ends of thetransmission line portion 10 or a first ground conductor 20 and thesecond ground conductor 30 may be exposed to the outside. In addition,the coaxial connectors 61 may be installed on different surfaces. Forexample, the coaxial connector 61 at one end may be installed on theside of the first principal surface, and the coaxial connector 61 at theother end may be installed on the side of the second principal surface.

As illustrated in FIGS. 1A to 1C, the transmission line portion 10 hasan appearance in which a flat dielectric element 110 is sandwichedbetween a protective layer 120 and a protective layer 130 from bothsides in the thickness direction of the dielectric element 110.Specifically, the protective layer 130 is arranged to extend generallyover the entire surface of the dielectric element 110 on the side of thefirst principal surface which is an end surface of the dielectricelement 110 on one side in the thickness direction. The protective layer120 is arranged to extend generally over the entire surface of thedielectric element 110 on the side of the second principal surface whichis an end surface of the dielectric element 110 on the other side in thethickness direction.

A specific structure of the first region 100S and the second region 100Hconstituting the transmission line portion 10 will be described next.

The first region 100S includes the dielectric element 110 including afirst dielectric layer 111 and a second dielectric layer 112. Thedielectric element 110 is a flexible flat plate having a predeterminedthickness. The dielectric element 110 is elongated in a first directionalong a flat-plate surface, and has a predetermined width in a directionalong the flat-plate surface and perpendicular or substantiallyperpendicular to the longitudinal direction (first direction). Thedielectric element 110 is preferably made of a flexible material such aspolyimide or a liquid crystal polymer, for example.

The signal conductor 40 has a flat-film shape, and is provided betweenthe first dielectric layer 111 and the second dielectric layer 112. Thesignal conductor 40 is preferably located generally in the center of thedielectric element 110 in the width direction. The width of the signalconductor 40 is smaller than the width of the dielectric element 110.More specifically, the width of the signal conductor 40 is smaller thanthe gap, in the width direction, between elongated conductors 21 and 22constituting the first ground conductor 20 to be discussed later. Thesignal conductor 40 is preferably located in the middle of thedielectric element 110 in the thickness direction. The position of thesignal conductor 40 in the thickness direction is set such that acharacteristic impedance desired for the transmission line portion 10 isobtained. In other words, the thickness of first dielectric layer 111and the second dielectric layer 112 is set such that a characteristicimpedance desired for the transmission line portion 10 is obtained. Thesignal conductor 40 preferably is made of a highly electricallyconductive material such as copper (Cu), for example.

The first ground conductor 20 is provided on the first principal surface(surface on the side of the first dielectric layer 111) of thedielectric element 110. However, if desired, the first ground conductor20 may be provided in the dielectric element 110. The first groundconductor 20 includes the elongated conductors 21 and 22 and bridgeconductors 23. The first ground conductor 20 preferably is also made ofa highly electrically conductive material such as copper (Cu), forexample.

The elongated conductors 21 and 22 have an elongated shape to extendalong the longitudinal direction of the dielectric element 110. Theelongated conductor 21 is preferably located at one end of thedielectric element 110 in the width direction. The elongated conductor22 is preferably located at the other end of the dielectric element 110in the width direction. The elongated conductors 21 and 22 are arrangedwith a predetermined gap therebetween along the width direction of thedielectric element 110.

The bridge conductors 23 are configured to extend in the width directionof the dielectric element 110. A plurality of bridge conductors 23 arearranged at intervals along the longitudinal direction of the dielectricelement 110. Thus, opening portions 24 are located between the bridgeconductors 23 as seen in the direction perpendicular or substantiallyperpendicular to the first principal surface (as seen along thethickness direction).

In this way, the first ground conductor 20 preferably has a ladder shapeto extend in the longitudinal direction.

The second ground conductor 30 is provided on the second principalsurface (surface on the side of the second dielectric layer 112) of thedielectric element 110. The second ground conductor 30 is arranged toextend generally over the entire surface of the dielectric element 110.The second ground conductor is also preferably made of a highlyelectrically conductive material such as copper (Cu), for example.

The first ground conductor 20 and the second ground conductor 30 areconnected to each other by connection conductors 50. The connectionconductors 50 are so-called electrically conductive via conductors, andpenetrate the dielectric element 110 in the thickness direction. Theconnection conductors 50 are provided at positions of the first groundconductor 20 at which the elongated conductors 21 and 22 and the bridgeconductors 23 are connected to each other.

In the thus configured first region 100S of the transmission lineportion 10, the signal conductor 40 provided in the dielectric element110 is sandwiched between the first ground conductor 20 and the secondground conductor 30. Thus, a so-called tri-plate transmission line isprovided. In this case, the second ground conductor 30 which is asolidly filled conductor defines and serves as a reference ground, andthe first ground conductor 20 with a ladder shape defines and serves asan auxiliary ground.

In the first region 100S, opening portions in which no conductor isprovided are arranged at predetermined intervals along the longitudinaldirection. Thus, for the first region 100S, the proportion of theconductor to the dielectric element 110 is lowered and the thickness ofthe dielectric element is reduced compared to a case where the firstground conductor is a solidly filled electrode as with the second groundconductor. Thus, the first region 100S is flexible, and is capable ofbeing easily bent at a predetermined angle in the thickness directionwith respect to a flat-plate surface of the flat cable.

The second region 100H includes a dielectric element 110H including afirst dielectric layer 111H and a second dielectric layer 112H. Thedielectric element 110H is a flexible flat plate having a predeterminedthickness. The dielectric element 110H is preferably wider than thedielectric element 110 of the first region 100S. The width of thedielectric element 110H may be about the same as the width of thedielectric element 110 of the first region 100S.

The dielectric element 110H is preferably made of a flexible materialsuch as polyimide or a liquid crystal polymer, for example. The firstdielectric layer 111H is preferably integral with the first dielectriclayer 111 of the first region 100S. The second dielectric layer 112H ispreferably integral with the second dielectric layer 112 of the firstregion 100S. That is, the dielectric element 110H of the second region100H is preferably integral with the dielectric element 110 of the firstregion 100S.

The second region 100H includes a meandering conductor 40H. Themeandering conductor 40H is located between the first dielectric layer111H and the second dielectric layer 112. Both ends of the meanderingconductor 40H are connected to the signal conductor 40 of the firstregion 100S connected to the second region 100H. The meanderingconductor 40H defines and functions as an inductor connected in serieswith a transmission line in a distributed constant circuit. The lengthof the meandering conductor 40H is set such that the phase of ahigh-frequency signal transmitted through the meandering conductor 40His rotated by a desired phase amount. Use of such an inductor for adistributed constant circuit provides a phase adjustment circuit withgood frequency characteristics that causes no variation in inductanceaccording to the frequency. Thus, a transmission signal is transmittedwith a low loss even in the case where the transmission signal has apredetermined frequency band.

The second region 100H includes a first ground conductor 20H. The firstground conductor 20H is provided on the first principal surface (surfaceon the side of the first dielectric layer 111H) of the dielectricelement 110H. The first ground conductor 20H is arranged to extendgenerally over the entire first principal surface of the dielectricelement 110H. That is, the first ground conductor 20H is a solidlyfilled conductor in the second region 100H. The first ground conductor20H is also made of a highly electrically conductive material such ascopper (Cu), for example. The first ground conductor 20H is preferablyintegral with the first ground conductor 20 of the first region 100S.

The second region 100H includes a second ground conductor 30H. Thesecond ground conductor 30H is provided on the second principal surface(surface on the side of the second dielectric layer 112H) of thedielectric element 110H. The second ground conductor 30H is arranged toextend generally over the entire second principal surface of thedielectric element 110H. That is, the second ground conductor 30H is asolidly filled conductor in the second region 100H. The second groundconductor 30H is also preferably made of a highly electricallyconductive material such as copper (Cu), for example. The second groundconductor 30H is preferably integral with the second ground conductor 30of the first region 100S.

The first ground conductor 20H and the second ground conductor 30H areconnected to each other by connection conductors 50H. The connectionconductors 50H are so-called electrically conductive via conductors, andpenetrate the dielectric element 110H in the thickness direction. Aplurality of connection conductors 50H are provided in regions excludinga region in which the meandering conductor 40H is arranged to define thesecond region 100H is viewed in plan.

The thus configured second region 100H defines and functions as a phaseadjustment circuit that rotates the phase of a high-frequency signal bya predetermined phase amount when the high-frequency signal istransmitted between adjacent first regions 100S connected to each other.Thus, a high-frequency signal input to the flat cable 60 is output afterbeing adjusted to a predetermined phase. The second region 100H may beprovided with a coil-shaped inductor pattern or capacitor pattern, andmay be configured to define and function as a filter circuit such as alow-pass filter or a high-pass filter. In the case where a coil-shapedinductor pattern is provided, the impedance of the second region 100H iseasily increased compared to the impedance at positions at which thebridge conductors 23 are located.

Further, the second region 100H according to the present preferredembodiment includes third ground conductors 41. The third groundconductors 41 are provided between the first dielectric layer 111H andthe second dielectric layer 112H. In other words, the third groundconductors 41 are provided at the same position as the meanderingconductor 40H along the thickness direction of the dielectric element110H. The third ground conductors 41 are provided at a predetermineddistance from the meandering conductor 40H to extend in parallel orsubstantially in parallel with the meandering conductor 40H. The thirdground conductors 41 are connected to the first ground conductor 20H andthe second ground conductor 30H by the connection conductors 50H. Withsuch a configuration, the characteristics of the phase adjustmentcircuit implemented by the second region 200H are capable of being sethighly precisely. Thus, a predetermined amount of phase rotation iscapable of being provided to a high-frequency signal with higherprecision. Further, the meandering conductor 40H is shielded from theexternal environment. Thus, leakage of extraneous radiation generated bythe meandering conductor 40H to the outside is significantly reduced orprevented.

With the configuration according to the present preferred embodiment, inaddition, in the second region 100H, the meandering conductor 400H issandwiched between solidly filled conductors, and thus the proportion ofthe conductor to the dielectric element is increased. Thus, the secondregion 100H is harder than the first region 100S, and is difficult to bebent. With the configuration according to the preferred embodimentdescribed above, further, the width of the second region 100H is largerthan the width of the first region 100S, which makes the second region100H harder and even more difficult to be bent.

When such a configuration is used, the transmission line portion 10 ofthe flat cable 60 has the following distribution characteristics ofcharacteristic impedance along the longitudinal direction. FIGS. 4A and4B are each a graph illustrating the distribution characteristics ofcharacteristic impedance along the longitudinal direction of thetransmission line portion 10 of the flat cable 60 according to thepresent preferred embodiment. FIG. 4A illustrates a state with no bend.FIG. 4B illustrates a state in which the first region 100S is bent.

In the first region 100S, the characteristic impedance is varied incycles matching the interval of arrangement of the bridge conductors 23.Specifically, the characteristic impedance becomes minimum with the Cproperty enhanced at positions of the bridge conductors 23 along thelongitudinal direction, at which the bridge conductors 23 and the signalconductor 40 face each other in the thickness direction. Thecharacteristic impedance becomes maximum with the L property enhancedmost at positions between the bridge conductors 23 along thelongitudinal direction, in other words, at the center positions of theopening portions 24 along the longitudinal direction. Thus, thevariation width of the characteristic impedance, which is the differencebetween the maximum value and the minimum value of the characteristicimpedance, in the first region 100S is ΔR0.

In the second region 100H, the meandering conductor 40H is preferablyused, and thus the characteristic impedance becomes maximum with the Lproperty enhanced most at the middle position in the extending directionof the meandering conductor 40H. On the other hand, the characteristicimpedance becomes minimum with the C property enhanced at the boundaryposition between the second region 100H and the first region 100S, atwhich the signal conductor 40 and the first ground conductor 20H faceeach other. Thus, the variation width of the characteristic impedance,which is the difference between the maximum value and the minimum valueof the characteristic impedance, in the second region 100H is ΔR1.

In this event, the length of the meandering conductor 40H can be set asappropriate to make the maximum value of the characteristic impedance inthe second region 100H larger than the maximum value of thecharacteristic impedance in the first region 100S. Thus, the variationwidth ΔR1 of the characteristic impedance in the second region 100H ispreferably larger than the variation width ΔR0 of the characteristicimpedance in the first region 100S.

Further, the variation width ΔR1 of the characteristic impedance in thesecond region 100H is preferably set to be larger than the maximum valueΔR0′ that the variation width ΔR0 of the characteristic impedance maytake when the first region 100S is curved or bent.

With such a configuration, even if the first region 100S is curved orbent and the characteristic impedance in the first region 100S isvaried, the maximum value of the variation width of the characteristicimpedance for the transmission line portion 10 is the variation widthΔR1 of the characteristic impedance in the second region 100H.

Thus, the characteristic impedance for the transmission line portion 10is hardly varied even if the first region 100S is curved or bent.

Thus, if the shape of various portions of the transmission line portion10 is set so as to obtain a desired characteristic impedance Z0 with thedistribution of the characteristic impedance in the first region 100Sand the characteristic impedance in the second region 100H taken intoconsideration in advance, the characteristic impedance for thetransmission line portion 10 is kept constant without being affected bya curve or a bend of the first region 100S. Thus, a flat cable with goodtransmission characteristics is achieved.

For example, the transmission characteristics are not degraded even ifthe flat cable 60 is utilized as curved at the first regions 100S asillustrated in FIGS. 1B and 1C, implementing a flat cable with goodtransmission characteristics.

The interval between the second regions 100H along the transmissiondirection, that is, the interval (“S” in FIG. 4A) between positions atwhich the absolute value of the impedance becomes largest, is preferablyshort. If the interval S is short, the frequency of a standing wavewhich affects the transmission-frequency characteristics of thetransmission line portion 10 is increased. Thus, thetransmission-frequency characteristics of the transmission line portion10 in the frequency band of the transmission signal are improved byadjusting the interval S to be short to achieve a high frequency that isdistant from the frequency of the high-frequency signal to betransmitted. That is, transmission characteristics with a low loss areprovided in the frequency band of the transmission signal.

With the configuration according to the present preferred embodiment,further, the phase of the high-frequency signal is rotated by apredetermined phase amount. Thus, the high-frequency signal is suppliedafter being adjusted to a more appropriate phase in the case where theflat cable is connected to a circuit element with output characteristicsthat differ in accordance with the phase such as a high-frequency poweramplifier. Thus, the characteristics (such as transmission signal outputcharacteristics, for example) of a high-frequency module in which theflat cable is used are improved.

The connection conductors 50H in the second region 100H discussed aboveare preferably larger in diameter than the connection conductors 50 inthe first region 100S. The number of the connection conductors 50Hprovided is preferably as large as possible. As the number of theconnection conductors 50H is larger, the hardness of the second region100H against a curve is enhanced.

The thus structured flat cable is manufactured as follows, for example.

First, a first insulating film to both surfaces of which copper isaffixed and a second insulating film to one surface of which copper isaffixed are prepared.

A first ground conductor 20 is formed by a patterning process at aportion on the side of the first principal surface of the firstinsulating film corresponding to the first region 100S. A first groundconductor 20H is formed by a patterning process at a portion on the sideof the first principal surface of the first insulating filmcorresponding to the second region 100H.

A signal conductor 40 is formed by a patterning process at a portion onthe side of the second principal surface of the first insulating filmcorresponding to the first region 100S. A meandering conductor 40H andthird ground conductors 41 are formed by a patterning process at aportion on the side of the second principal surface of the firstinsulating film corresponding to the second region 100H.

A plurality of sets of the first ground conductors 20 and 20H, thesignal conductor 40, the meandering conductor 40H, and the third groundconductors 41 are formed and arranged on the first insulating film.

A second ground conductor 30 is formed by a patterning process at aportion on the side of the second principal surface of the secondinsulating film corresponding to the first region 100S. A second groundconductor 30H is formed by a patterning process at a portion on the sideof the second principal surface of the second insulating filmcorresponding to the second region 100H. A plurality of the secondground conductors 30 and 30H are formed and arranged on the secondinsulating film.

The first insulating film and the second insulating film are affixed toeach other such that the first ground conductor 20 and the second groundconductor 30 face each other and the first ground conductor 20H and thesecond ground conductor 30H face each other. In this event, the firstinsulating film and the second insulating film are affixed to each othersuch that the signal conductor 40, the meandering conductor 40H, and thethird ground conductors 41 are disposed between the first insulatingfilm and the second insulating film. Thus, a plurality of complexes inwhich the first ground conductors 20 and 20H and the second groundconductors 30 and 30H are formed on both surfaces of a dielectricelement including the signal conductor 40, the meandering conductor 40H,and the third ground conductors 41 provided at the middle position inthe thickness direction can be obtained.

Individual transmission line portions 10 are cut out from the complexes.Protective layers 120 and 130 are formed on the transmission lineportions 10. Coaxial connectors 61 are installed via conversionpedestals 62 at both ends of the transmission line portion 10 in thelongitudinal direction, and on a surface of the transmission lineportion 10 on which the protective layer 130 is formed.

The flat cable 60 structured as discussed above can be used for thefollowing portable electronic device. FIG. 5A is a side sectional viewillustrating the configuration of a portable electronic device accordingto the first preferred embodiment of the present invention. FIG. 5B is aplan sectional view illustrating the configuration of the portableelectronic device.

A portable electronic device 1 includes a thin device housing 2. Circuitboards 3A and 3B and a battery pack 4 are disposed in the device housing2. A plurality of IC chips 5 and components 6 are mounted on surfaces ofthe circuit boards 3A and 3B. The circuit boards 3A and 3B and thebattery pack 4 are installed in the device housing 2 such that thebattery pack 4 is disposed between the circuit boards 3A and 3B as thedevice housing 2 is viewed in plan. The device housing 2 is preferablyas thin as possible, and thus the gap between the battery pack 4 and thedevice housing 2 is extremely narrow in the thickness direction of thedevice housing 2. Thus, a coaxial cable cannot be disposed in the gap.

However, the flat cable 60 described in the present preferred embodimentpreferably is disposed such that the thickness direction of the flatcable 60 and the thickness direction of the device housing 2 coincidewith each other to allow the flat cable 60 to pass between the batterypack 4 and the device housing 2. Thus, the circuit boards 3A and 3Bspaced from each other with the battery pack 4 disposed in the middleare connected to each other by the flat cable 60.

Further, even in the case where the position of connection of the flatcable 60 to the circuit boards 3A and 3B and a surface of the batterypack 4 for installation of the flat cable 60 are different in thethickness direction of the device housing 2 and the flat cable 60 iscurved to be connected, a high-frequency signal is transmitted whilesuppressing a transmission loss by using the structure according to thepresent preferred embodiment.

Next, a flat cable which is a high-frequency transmission line accordingto a second preferred embodiment of the present invention will bedescribed with reference to the drawings. FIG. 6 is an explodedperspective view illustrating the structure of a second region of atransmission line portion of the flat cable according to the secondpreferred embodiment of the present invention.

A transmission line portion 10 a of a flat cable 60 a according to thepresent preferred embodiment is different from the transmission lineportion 10 described in the first preferred embodiment in the structureof a second region 100HC, and otherwise preferably is the same as thetransmission line portion 10. Thus, only differences will be described.

The second region 100HC includes a flat-plate conductor 40HC providedbetween the first dielectric layer 111H and the second dielectric layer112H. The flat-plate conductor 40HC is arranged to extend generally overthe entire surface of the second region 100HC as seen along thethickness direction. The flat-plate conductor 40HC is a so-calledsolidly filled conductor.

A plurality of connection conductors 50H are arranged to connect betweenthe first ground conductor 20H and the second ground conductor 30H atpositions with no connection to the flat-plate conductor 40HC.

With such a configuration, the flat-plate conductor 40HC, the firstground conductor 20H and the second ground conductor 30H, and the firstand second dielectric layers 111H and 112H define and function as acapacitor connected between the transmission line and the ground.

Even with such a configuration, a phase adjustment circuit that rotatesthe phase of a high-frequency signal is provided.

With such a configuration, the transmission line portion 10 a of theflat cable 60 a has the following distribution characteristics ofcharacteristic impedance along the longitudinal direction. FIG. 7 is agraph illustrating the distribution characteristics of characteristicimpedance along the longitudinal direction of the transmission lineportion 10 a of the flat cable 60 a according to the second preferredembodiment of the present invention.

As in the first preferred embodiment, the variation width of thecharacteristic impedance, which is the difference between the maximumvalue and the minimum value of the characteristic impedance, in thefirst region 100S is ΔR0.

In the second region 100 HC, a capacitance is generated between thefirst ground conductor 20H and the second ground conductor 30H using theflat-plate conductor 40HC, which enhances the C property. Thus, thecharacteristic impedance is greatly reduced compared to that at theboundary position between the first region 100S and the second region200HC to become minimum. Thus, the variation width of the characteristicimpedance, which is the difference between the maximum value and theminimum value of the characteristic impedance, in the second region100HC is ΔR2, which is larger than the variation width ΔR0 in the firstregion 100S.

Further, the variation width ΔR2 of the characteristic impedance in thesecond region 100HC is preferably set to be larger than the maximumvalue ΔR0′ that the variation width ΔR0 of the characteristic impedancemay take when the first region 100S is curved or bent.

With such a configuration, even if the first region 100S is curved orbent and the characteristic impedance in the first region 100S isvaried, the maximum value of the variation width of the characteristicimpedance for the transmission line portion 10 a is the variation widthΔR2 of the characteristic impedance in the second region 100HC.

Thus, the characteristic impedance for the transmission line portion 10a is hardly varied even if the first region 100S is curved or bent.

Thus, if the shape of various portions of the transmission line portion10 a is set so as to obtain a desired characteristic impedance Z0 withthe distribution of the characteristic impedance in the first region100S and the characteristic impedance in the second region 100HC takeninto consideration in advance, the characteristic impedance for thetransmission line portion 10 a can be kept constant without beingaffected by a curve or a bend of the first region 100S. Thus, a flatcable with good transmission characteristics is provided.

In the configuration according to the present preferred embodiment,further, the first and second dielectric layers 111H and 112H aresandwiched by the first ground conductor 20H, the flat-plate conductor40HC, and the second ground conductor 30H, which are solidly filledelectrodes, in the thickness direction. Thus, the second region 100HC ismore resistant to being curved in the thickness direction. Thus, it ismore difficult to curve the second region 100HC, and variations incharacteristic impedance in the second region 100HC are significantlyreduced or prevented.

The interval between the second regions 100H along the transmissiondirection, that is, the interval (“S” in FIG. 7) between positions atwhich the absolute value of the impedance becomes largest, is preferablyshort. If the interval S is short, the frequency of a standing wavewhich affects the transmission-frequency characteristics of thetransmission line portion 10 a is increased. Thus, thetransmission-frequency characteristics of the transmission line portion10 a in the frequency band of the transmission signal is improved byadjusting the interval S to be short to achieve a high frequency that isdistant from the frequency of the high-frequency signal to betransmitted. That is, transmission characteristics with a low loss areimplemented in the frequency band of the transmission signal.

Next, a flat cable which is a high-frequency transmission line accordingto a third preferred embodiment of the present invention will bedescribed with reference to the drawings. FIG. 8 is an explodedperspective view illustrating the structure of a second region of atransmission line portion of the flat cable according to the thirdpreferred embodiment of the present invention.

A transmission line portion 10 b of a flat cable 60 b according to thepresent preferred embodiment is different from the transmission lineportion 10 a described in the second preferred embodiment in thestructure of the second region 100HC and the structure of the firstdielectric layer, and otherwise preferably is the same as thetransmission line portion 10 a. Thus, only differences will bedescribed.

First ground conductors 20Hb1 and 20Hb2 are provided on the firstdielectric layer 111H of the second region 100HC. The first groundconductors 20Hb1 and 20Hb2 are flat-plate conductors. The first groundconductors 20Hb1 and 20Hb2 are spaced from each other with apredetermined gap 200Hb therebetween. In other words, the first groundconductor located in the second region 100HC is divided at anintermediate position along the transmission direction.

The first ground conductor 20Hb1 is connected to the elongatedconductors 21 and 22 provided on one of the first dielectric layers 111connected to the first dielectric layer 111H. The first ground conductor20Hb2 is connected to the elongated conductors 21 and 22 provided on theother first dielectric layer 111 connected to the first dielectric layer111H.

The first ground conductors 20Hb1 and 20Hb2 face the flat-plateconductor 40HC with the first dielectric layer 111H interposedtherebetween.

The first ground conductors 20Hb1 and 20Hb2 are connected to the secondground conductor 30H via the individual connection conductors 50H. Aplurality of connection conductors 50H are provided at positions with noconnection to the flat-plate conductor 40HC.

With such a configuration, the first ground conductor 20Hb1 and thesecond ground conductor 30H constitute a capacitor, the first groundconductor 20Hb and the second ground conductor 30H constitute acapacitor, and the flat-plate conductor 40HC and the second groundconductor 30H constitute a capacitor. The capacitors define and functionas a capacitor connected between the transmission line and the ground.

Even with such a configuration, functions and effects similar to thoseof the preferred embodiments discussed above are achieved. In the casewhere the configuration according to the preferred embodiment is used,the capacitance of the capacitor connected between the transmission lineand the ground is capable of being adjusted by adjusting the shape ofthe gap 200Hb, in other words, by adjusting the shape of the firstground conductors 20Hb1 and 20Hb2.

In the present preferred embodiment, the first ground conductorpreferably is divided. However, the second ground conductor may bedivided.

Next, a flat cable which is a high-frequency transmission line accordingto a fourth preferred embodiment will be described with reference to thedrawings. FIG. 9 is an exploded perspective view illustrating thestructure of a second region of a transmission line portion of the flatcable according to the fourth preferred embodiment of the presentinvention.

A transmission line portion 10 c of a flat cable 60 c according to thepresent preferred embodiment is different from the transmission lineportion 10 a described in the second preferred embodiment in thestructure of the first dielectric layer and the structure of the secondregion 100HC, and otherwise preferably is the same as the transmissionline portion 10 a. Thus, only differences will be described.

The second dielectric layer preferably has a two-layer structure withdielectric layers 1121 and 1122. The dielectric layer 1121 is disposedon the side of the first dielectric layer 111. The signal conductor 40is located in the first region on the dielectric layer 1121 on the sideof the first dielectric layer 111. The flat-plate conductor 40HC islocated in the second region on the dielectric layer 1121 on the side ofthe first dielectric layer 111. A flat-plate conductor 42 is located inthe second region on the dielectric layer 1122 on the side of thedielectric layer 1121. The flat-plate conductor 42 is arranged so as toface the flat-plate conductor 40HC over a predetermined area. Theflat-plate conductor 42 is connected to the second ground conductor 30Hvia the connection conductor 50H.

No conductors (first ground conductors) are provided on the firstdielectric layer 111H of the second region 100HC. The bridge conductors23 which connect between the elongated conductors 21 and 22 are providedat end portions, on the side of the second region 100HC, on the firstdielectric layer 111 of the first regions 100S which interpose thesecond region 100HC.

With such a configuration, the flat-plate conductor 40HC and theflat-plate conductor 42 (connected to the second ground conductor 30H)constitute a capacitor. The capacitor defines and functions as acapacitor connected between the transmission line and the ground.

With such a configuration, the capacitance generated between theflat-plate conductor 40HC and the second ground conductor is increasedcompared to a case where the flat-plate conductor 42 is not formed. Thecapacitance is capable of being adjusted by adjusting the shape of theflat-plate conductor 42. Further, the capacitance is capable of beingadjusted by adjusting the thickness of the dielectric layer 1121 betweenthe flat-plate conductors 40HC and 42.

In the present preferred embodiment, the flat-plate conductor 42 and thesecond ground conductor 30HC are connected to each other by theconnection conductor 50H. However, the flat-plate conductor 40HC and theflat-plate conductor 42 may be connected to each other by a connectionconductor, and the connection conductor between the flat-plate conductor42 and the second ground conductor 30HC may be omitted. In this case,the capacitance is capable of being adjusted in accordance with thethickness of the dielectric layer 1122.

Next, a flat cable which is a high-frequency transmission line accordingto a fifth preferred embodiment will be described with reference to thedrawings. FIG. 10 is an exploded perspective view illustrating thestructure of a second region of a transmission line portion of the flatcable according to the fifth preferred embodiment of the presentinvention.

A transmission line portion 10 d of a flat cable 60 d according to thepresent preferred embodiment is different from the transmission lineportion 10 a described in the second preferred embodiment in thestructure of the second region 100HC, and otherwise preferably is thesame as the transmission line portion 10. Thus, only differences will bedescribed.

The flat-plate conductor 42 is provided on the first dielectric layer111H in the second region 100HC. The flat-plate conductor 42 is arrangedto extend generally over the entire surface of the first dielectriclayer 111H excluding a region with a predetermined width along the outerperiphery of the first dielectric layer 111H. The flat-plate conductor42 is separated from the elongated conductors 21 and 22 formed on thefirst dielectric layer 111 of the first region 100S. A routing conductor421 is connected to the flat-plate conductor 42. The routing conductor421 is provided on the first dielectric layer 111H of the second region100 HC.

The bridge conductors 23 which connect between the elongated conductors21 and 22 are provided at end portions, on the side of the second region100HC, on the first dielectric layer 111 of the first regions 100S whichinterpose the second region 100HC.

A flat-plate conductor 40HCd is provided on the second dielectric layer112H in the second region 100HC. The flat-plate conductor 40HCd is alsoarranged to extend generally over the entire surface of the seconddielectric layer 112H excluding a region with a predetermined widthalong the outer periphery of the second dielectric layer 112H. Theflat-plate conductor 40HCd is connected to only the signal conductor 40formed on one of the first regions 100S, and separated from the signalconductor 40 on the other first region 100S. The side on which theflat-plate conductor 42 is separated from the linear conductor 100 isthe same as the side on which the routing conductor 421 is formed on thefirst dielectric layer 111H.

The signal conductor 40 on the separated side extends to the secondregion 100HC. The signal conductor 40 extending to the second region100HC and the routing conductor 421 of the first dielectric layer 111Hare connected to each other by a connection conductor 52.

With such a configuration, the flat-plate conductors 40HCd and 42constitute a capacitor provided across the first dielectric layer 111H.The flat-plate conductor 40HCd is connected to one of the signalconductors 40, and the flat-plate conductor is connected to the othersignal conductor 40. Thus, the capacitor defines and functions as acapacitor connected in series with the signal transmission line.

Further, a second ground conductor 30Hd of the second region 100HC isopen except for a region with a predetermined width along the outerperiphery discussed above. With such a configuration, the flat-platesurfaces of the flat-plate conductor 40HCd and the second groundconductor 30Hd do not face each other. Thus, almost no capacitivecoupling is caused between the flat-plate conductor 40HCd and the secondground conductor 30Hd. Thus, in the case where only a series capacitoris required, such an opening is preferably provided. In the case where aparallel capacitor is also required, an opening is preferably notprovided.

Next, a flat cable which is a high-frequency transmission line accordingto a sixth preferred embodiment of the present invention will bedescribed with reference to the drawings. FIG. 11 is an explodedperspective view illustrating the structure of a second region of atransmission line portion of the flat cable according to the sixthpreferred embodiment of the present invention.

A transmission line portion 10 e of a flat cable 60 e according to thepreferred embodiment is different from the transmission line portion 10a described in the second preferred embodiment in the structure of thesecond region 100HC, and otherwise preferably is the same as thetransmission line portion 10 a. Thus, only differences will bedescribed.

A pair of comb-teeth conductors 40HCe1 and 40HCe2 are provided on thesecond dielectric layer 112H in the second region 100HC. The comb-teethconductors 40HCe1 and 40HCe2 are connected to different signalconductors 40. The comb-teeth conductors 40HCe1 and 40HCe2 are disposednot to be connected to each other. In other words, conductive fingers ofthe comb-teeth conductors 40HCe1 and 40HCe2 are disposed atpredetermined intervals in the width direction of the second dielectriclayer 112H.

With such a configuration, a capacitance is generated between theconductive fingers of the comb-teeth conductors 40HCe1 and 40HCe2 sothat the comb-teeth conductors 40HCe1 and 40HCe2 define and function asa capacitor. The capacitance is generated through coupling on the seconddielectric layer 112H, that is, within an identical plane, and thus noteasily varied under the influence of a conductor disposed outside theplane.

In the configuration, in addition, the capacitance is determined inaccordance with the length of the conductive fingers provided oppositeto each other, and therefore not affected by the thickness of thedielectric layer between the conductors constituting the capacitor.Thus, the design capacitance is achieved reliably and accurately.

With the configuration, in addition, the shielding performance againstan external electromagnetic field is improved if the comb-teethconductors 40HCe1 and 40HCe2 constituting the capacitor are interposedbetween the first ground conductor 20H and the second ground conductor30H which are solidly filled conductors.

Next, a flat cable which is a high-frequency transmission line accordingto a seventh preferred embodiment of the present invention will bedescribed with reference to the drawings. FIG. 12 is an explodedperspective view illustrating the structure of a second region of atransmission line portion of the flat cable according to the seventhpreferred embodiment of the present invention.

A transmission line portion 10 f of a flat cable 60 f according to thepresent preferred embodiment is different from the transmission lineportion 10 c described in the fourth preferred embodiment in thestructure of the first dielectric layer and the structure of the secondregion 100HC, and otherwise preferably is the same as the transmissionline portion 10 c. Thus, only differences will be described.

The second dielectric layer preferably has a two-layer structure withdielectric layers 1121 and 1122. The dielectric layer 1121 is disposedon the side of the first dielectric layer 111. The signal conductor 40is located in the first region 100S on the dielectric layer 1121 on theside of the first dielectric layer 111. Flat-plate conductors 40HCf1 and40HCf2 are located in the second region 100HC on the dielectric layer1121 on the side of the first dielectric layer 111. The flat-plateconductors 40HCf1 and 40HCf2 are separated from each other with apredetermined gap therebetween. The flat-plate conductors 40HCf1 and40HCf2 are connected to different signal conductors 40.

A flat-plate conductor 42 is located in the second region 100HC on thedielectric layer 1122 on the side of the dielectric layer 1121. Theflat-plate conductor 42 is arranged so as to face the flat-plateconductors 40HCf1 and 40HCf2 over a predetermined area.

No conductors (first ground conductors) are provided on the firstdielectric layer 111H of the second region 100HC. The bridge conductors23 which connect between the elongated conductors 21 and 22 are providedat end portions, on the side of the second region 100HC, on the firstdielectric layer 111 of the first regions 100S which interpose thesecond region 100HC.

With such a configuration, the flat-plate conductors 40HCf1 and 42 faceeach other across the dielectric layer 1121 to constitute a capacitor.In addition, the flat-plate conductors 40HCf2 and 42 face each otheracross the dielectric layer 1121 to constitute a capacitor. Since theflat-plate conductors 40HCf1 and 40HCf2 are connected to differentsignal conductors 40, the capacitors define and function as twocapacitors connected in series with the signal transmission line.

Further, a second ground conductor 30Hf of the second region 100HC isopen except for a region with a predetermined width along the outerperiphery discussed above. With such a configuration, the flat-platesurfaces of the flat-plate conductor 42 and the second ground conductor30Hf do not face each other. Thus, almost no capacitive coupling iscaused between the flat-plate conductor 42 and the second groundconductor 30Hf. Thus, in the case where only a series capacitor isrequired, such an opening is preferably provided. In the case where aparallel capacitor is also required, an opening is preferably notprovided.

In the preferred embodiments discussed above, the second regions 100Hand 100HC are wider than the first region 100S. However, the flat cablemay have a constant width as illustrated in FIG. 13. FIG. 13 is aperspective view illustrating the appearance of a flat cable with aconstant width. As illustrated in FIG. 13, a transmission line portion10′ of a flat cable 60′ includes first regions 100S and second regions100H′ connected alternately. The first regions 100S and the secondregions 100H′ preferably have the same width as each other. Such aconfiguration with a constant width may be adopted if the rigidity ofthe second regions 100H′ is higher than the rigidity of the firstregions 100S.

In the preferred embodiments discussed above, the width of the openingportion 24 in the first region 100S preferably is constant, for example.However, the opening portion may be successively widened from an endportion connected to the bridge conductor toward the center in thelongitudinal direction of the opening portion. Thus, abrupt variationsin characteristic impedance in the first region are significantlyreduced or prevented so as to further reduce or prevent a transmissionloss.

In the preferred embodiments discussed above, the width of the signalconductor 40 in the first region 100S preferably is constant, forexample. However, the signal conductor may be widened at a portionfacing the opening portion. In this event, the signal conductor is notwidened to such a degree that the signal conductor overlaps theelongated conductors as the flat cable is seen in the thicknessdirection. Thus, the high-frequency resistance of the signal conductoris reduced to reduce the conductor loss of the flat cable.

Although not specifically described in the preferred embodimentsdiscussed above, a single second region may be provided rather than aplurality thereof. In the case where a plurality of second regions areprovided, the second regions preferably have generally the samevariation width of the characteristic impedance. Thus, the influence ofthe characteristic impedance in the second region on the flat cable isfurther increased to achieve a further stable characteristic impedancefor the flat cable.

In the preferred embodiments discussed above, a phase adjustment circuitfor the second region is preferably defined by a distributed constantcircuit, for example. However, the phase adjustment circuit for thesecond region may be implemented by a lumped-constant circuit element,that is, a surface mount coil or a surface mount capacitor. In the casewhere a surface mount coil is used, for example, the signal conductor 40is extended to the second region. The signal conductor 40 is cut in thesecond region. Two lands are provided on a surface of the firstdielectric layer 111H, and connected to the two cut ends of the signalconductor 40 by a via conductor or the like. A surface mount coil ismounted to the two lands.

In the preferred embodiments discussed above, a tri-plate transmissionline in which a signal conductor is disposed between a first groundconductor and a second ground conductor preferably is used. However, theconfiguration discussed above may also be applied to a microstriptransmission line from which a second ground conductor has been omitted.

In the preferred embodiments discussed above, the high-frequencytransmission line is preferably in the form of a flat cable. However,the present invention is not limited to a flat cable. That is, preferredembodiments of the present invention may be used for a high-frequencytransmission line that constitutes a portion of an RF circuit board suchas an antenna front-end module.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing from the scopeand spirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

What is claimed is:
 1. A high-frequency transmission line comprising: aflexible flat dielectric element elongated in one direction, thedielectric element having a predetermined width in a directionperpendicular or substantially perpendicular to a longitudinal directionand having a predetermined thickness; a signal conductor disposed in thedielectric element and configured to extend along the longitudinaldirection; and a first ground conductor opposing the signal lineconductor and extending in the longitudinal direction; wherein at leastone of the signal conductor and the first ground conductor has acharacteristic impedance cyclically varied along the longitudinaldirection, and includes a first region and a second region withdifferent variation widths of the characteristic impedance; and thevariation width of the characteristic impedance in the second region islarger than the variation width of the characteristic impedance in thefirst region.
 2. The high-frequency transmission line according to claim1, wherein in the first region, the first ground conductor is configuredto extend along the longitudinal direction, and includes two elongatedconductors disposed at both ends in the width direction with a spacebetween each other and configured to extend in the longitudinaldirection, and a plurality of bridge conductors that connect between thetwo elongated conductors at intervals along the longitudinal direction.3. The high-frequency transmission line according to claim 2, whereinthe second region is configured to rotate a phase of a high-frequencysignal by a predetermined amount through transmission between an inputend and an output end.
 4. The high-frequency transmission line accordingto claim 3, wherein in the second region, the signal conductor is aconductor line with a length that rotates the phase of thehigh-frequency signal by the predetermined amount and the first groundconductor is a conductor arranged to extend over an entirety orsubstantially the entirety of one end surface of the dielectric element.5. The high-frequency transmission line according to claim 3, furthercomprising: a third ground conductor provided in the second region andlocated at or substantially at a same position as the signal conductorin a thickness direction, the third ground conductor being configured toextend in parallel or substantially in parallel with the signalconductor with a constant or a substantially constant gap therebetween,and the third ground conductor being connected to the first groundconductor.
 6. The high-frequency transmission line according to claim 1,wherein a width of the second region is larger than a width of the firstregion.
 7. The high-frequency transmission line according to claim 1,wherein the second region is provided at a plurality of locations alongthe longitudinal direction, and a plurality of the second regions havethe same or substantially the same variation width of the characteristicimpedance.
 8. The high-frequency transmission line according to claim 1,further comprising a second ground conductor opposite to the firstground conductor with the signal conductor interposed therebetween. 9.The high-frequency transmission line according to claim 1, furthercomprising a connector member configured to be connected to the signalconductor provided at at least one end in the longitudinal direction.10. An electronic device comprising: the high-frequency transmissionline according to claim 1; a plurality of circuit boards connected toeach other by the high-frequency transmission line; and a housing thathouses the circuit boards.
 11. The electronic device according to claim10, wherein the high-frequency transmission line is bent at at least onelocation in the first region.
 12. The electronic device according toclaim 10, wherein in the first region, the first ground conductor isconfigured to extend along the longitudinal direction, and includes twoelongated conductors disposed at both ends in the width direction with aspace between each other and configured to extend in the longitudinaldirection, and a plurality of bridge conductors that connect between thetwo elongated conductors at intervals along the longitudinal direction.13. The electronic device according to claim 12, wherein the secondregion is configured to rotate a phase of a high-frequency signal by apredetermined amount through transmission between an input end and anoutput end.
 14. The electronic device according to claim 13, wherein inthe second region, the signal conductor is a conductor line with alength that rotates the phase of the high-frequency signal by thepredetermined amount and the first ground conductor is a conductorarranged to extend over an entirety or substantially the entirety of oneend surface of the dielectric element.
 15. The electronic deviceaccording to claim 13, further comprising: a third ground conductorprovided in the second region and located at or substantially at a sameposition as the signal conductor in a thickness direction, the thirdground conductor being configured to extend in parallel or substantiallyin parallel with the signal conductor with a constant or a substantiallyconstant gap therebetween, and the third ground conductor beingconnected to the first ground conductor.
 16. The electronic deviceaccording to claim 11, wherein a width of the second region is largerthan a width of the first region.
 17. The electronic device according toclaim 11, wherein the second region is provided at a plurality oflocations along the longitudinal direction, and a plurality of thesecond regions have the same or substantially the same variation widthof the characteristic impedance.
 18. The electronic device according toclaim 11, further comprising a second ground conductor opposite to thefirst ground conductor with the signal conductor interposedtherebetween.
 19. The electronic device according to claim 11, furthercomprising a connector member configured to be connected to the signalconductor provided at at least one end in the longitudinal direction.